NSF Award
2120326
This research is designed to understand how sub-microscopic amounts of water, in minerals, can affect their strength. The overall effect of this tiny amount of water, in rocks, can have a big significance to how plates interact. For example, the Indian plate is colliding with the Asian plate to produce the Himalayas, but the strong Indian plate is not deforming while the weaker Asian plate is folding, faulting, experiencing earthquakes, and rising to produce the tallest mountains on Earth. The uplift of the Tibetan Plateau (which is five times the area of France) to an elevation of over 3 miles high (4.8 km) is remarkable. One possibility for this huge difference in plate strength is that deep rocks of the Indian plate are “dry” (no sub-microscopic water) whereas deep rocks of the Asian plate are “wet” (they contain some sub-microscopic water). Therefore, it is important to understand how and when this tiny amount of water gets into the rocks. This research project involves collecting rocks, in the mountains of California, along a 5 km transect where it is already known that rocks undergo the transition from strong to weak behavior. Detailed chemical analyses with advanced micro-imaging techniques will be conducted to determine exactly how and where along the transect the water gets into and out of the effected minerals. This research will support the education of one post-doctoral researcher, 4-6 undergraduate researchers, as well as 12 inner city Detroit High School (César Chávez Academy High School) students and three High School teachers who will travel to California with the research team to sample and learn about how water affects mountain building and plate tectonics. The effects of water on shaping this part of Earth are visually evident (previous glaciers, pre-historic and historic lake beds) and the effects of climate change, drought and fires, and human intervention (water piped to Los Angeles) will also be examined. <br/><br/><br/>The goal of this project is to determine if grain boundary migration allows water to enter the quartz crystal lattice and cause weakening. Oxygen isotope data will be used as the main proxy for tracking water infiltration. Harkless Formation quartzite samples will be collected along a 5 km transect, oriented perpendicular to the contact with the Eureka Valley-Joshua Flat-Beer Creek pluton in the White-Inyo Range of California. Contact metamorphism is first observed at 2.9 km from the pluton and concordancy occurs abruptly at 1.1 km where the Harkless folds 90° and is intensely attenuated. This abrupt transition is assumed to be a “rolling hinge”, that progressed outward as the pluton expanded during emplacement. Samples will be collected across this transition between regional structures and the forceful concordance of country rocks with the intrusion. Standard petrography and scanning electron microscopy techniques with advanced analysis including electron backscatter diffraction and cathodoluminescence, will be used to document; 1) where exactly grain boundary migration begins, where it becomes pervasive and, 2) where the crystallographic preferred orientation begins to develop and how it develops across the transition. Using a focused Secondary Ion Mass Spectrometer beam with a small spot size (6-10µm pit size), grain boundaries will be analyzed between unmigrated and migrated parts of the grains to determine the oxygen isotope signatures for tracking water infiltration and delineating the potential sources of this water. Fourier-transform infrared spectroscopy will also be used to document the overall concentrations of water (OH) on a microscopic scale in associated minerals. Trace element data will be input into several TitaniQ models to determine the temperatures during deformation and the overall Ti diffusional partial re-equilibration during the cooling history. This overall strategy will help correlate rock/mineral strength and deformation behavior with sub-microscopic water phenomena in minerals from intense heating and pressure during mountain building processes.<br/><br/>This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.
